A tubular product in the present invention includes tubes in which the inner circumference or outer circumference of the cross-section thereof has a shape other than a circular shape, for example, tubes having special shapes including, as a specific example, a finned tube in which fins protruded from the inner peripheral surface or the outer peripheral surface lie along the tube axis direction, and being used as a heat transfer tube in an ethylene plant, etc.
Examples of the method for automatically measuring the outer diameter and the wall thickness of a tubular product include a method that utilizes a contact type measurement instrument, one that utilizes a laser, one that utilizes a camera, and the like.
The method that utilizes a contact type measurement instrument includes, for example, a method described in Patent Literature 1, in which the measurement apparatus tends to be huge in scale.
In the method that utilizes a laser, since a special mechanism for rotating the tubular product or the laser apparatus (including a light receiving element) to measure the whole circumference of the tubular product, the measurement apparatus will be huge in scale, similarly to the method that utilizes a contact type measurement instrument. Therefore, in these two methods described above, it is difficult to measure the wall thickness of a longer-length tubular product.
In contrast to these methods, the method that utilizes a camera is capable of dimensional inspection of a tubular product with a simple configuration, and moreover is highly promising as a technique which can be easily automated. Examples of the prior art for measuring the outer diameter and wall thickness of a tube by utilizing a camera include techniques disclosed in Patent Literatures 2 to 4 listed below.
However, the apparatuses disclosed in each Patent Literature have various problems. For example, when the dimensional measurement apparatus disclosed in Patent Literature 2 is used, halation occurs on the image acquired by the camera due to the reflection of light that is projected to the tube end face, making it difficult to distinguish the external and internal contours of the tube from the image. Further, since this apparatus projects light onto the inner peripheral surface as well as the end face of the tube, a significant difference in luminance hardly occurs between the tube end face and the inner peripheral surface on the image acquired by the camera, and thus it is difficult to distinguish the internal contour of the tube from the image.
The dimensional measurement apparatus disclosed in Patent Literature 3 also has difficulty in distinguishing the external and internal contours of the tube from the image acquired by the camera as with the dimensional measurement apparatus disclosed in Patent Literature 2. Further, the dimensional measurement apparatus disclosed in this can measure only a partial area of the tube along a circumferential direction. In order to measure the whole circumference of the tube, a special mechanism for rotating the tube or each camera (including each light source) about the central axis of the tube is necessary, and therefore the measurement apparatus will be huge in scale.
In the dimensional measurement method disclosed in Patent Literature 4, since it is necessary to dispose a camera and a light source with the tube being interposed therebetween, the measurement apparatus will be huge in scale. Therefore, it is difficult to measure a long-length tube.
Moreover, to ensure the quality of the tube, an inner surface inspection for detecting surface defects such as cracks and flaws which may be present on the inner peripheral surface of the tube is performed in addition to the dimensional inspection for measuring the outer diameter and the wall thickness of tube. Since, heretofore, the inner surface inspection of tube is performed through visual inspection by workers, there is a risk that defects may not be detected. For this reason, there is a need for automating the inner surface inspection of tube.